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December 1999

Dear Jane,

I have spent the most interesting few weeks working with archaeologists in a cave called Pin Hole at Creswell Crags. Its nothing like I expected.The TV programmes make excavations seem so quick but there we were for weeks digging, making notes and drawing.

Much of the cave had already been explored, most of it by an archaeologist called Leslie Armstrong in the 1920's and 1930's. He had done a good job for his day but left many questions unanswered. We started where he left off, looking at a cut through the depth of the deposits nearly four metres high. This section looked quite daunting as you could see many changes in colour which could be significant but they did not extend in nice horizontal layers like you see in text books. They were horribly complicated, dipping, rising, thinning out and disappearing in ways it was tricky to follow. How would we sort all this out and understand it?

It was awesome to realise that these deposits would take us back far into the Ice Age and would probably contain clues about how the natural world around Creswell had changed. Would you believe there is even well preserved pollen from the Ice Age. The rest of the plants have long since rotted away but, when the palaeobotanist came to take samples through the whole section, he told us that he could wash out the pollen grains then identify what plants they came from by looking at them through a microscope. He will be doing this and counting the grains of each type of plant in every layer. Then he will be able to say what the vegetation was like and how it changed as the climate got warmer or colder.

The work was tiring because we were in such a cramped space and had to be so careful, using just hand trowels and brushes. We had to record the position of all the objects we saw. These were mainly bones and teeth - some of them from animals which are now extinct. We were really fascinated by the skeleton of a baby hyaena which had died in its burrow. Particularly exciting were the occasional finds of stone tools which showed that people had used the cave

We did not throw anything away, but instead put all the dug sediment in bags to put through fine mesh sieves and sort in the good light of the Museum. This made sure that we did not miss the tiny bones and teeth of small animals such as voles and lemmings. These little creatures are so fussy about where they live and what they eat, they tell us a lot about the climate and vegetation around the cave at particular moments in time. Their remains got into the cave because they were captured and eaten by birds of prey which then regurgitated them in fur balls onto the cave floor where they were slowly buried.

I was told that in five years of digging about 30,000 finds came from just the top metre of deposit. I can well believe it. This is much more than survives from all the previous excavations in the cave but that is what modern techniques are all about. Even the grains of sediment tell a story. A sedimentologist spent a lot of time working with us, looking at the layers and how these changed. He also took samples which might be looked in a scanning electron microscope. Apparently, the shape, colour and texture of the grains and their geology can tell us how the cave filled up and under what climatic conditions.

For the moment, I can only tell you that the site was being used during the Last Ice Age between about 70,000 and 10,000 years ago. This we can tell from the plants and animals but other scientists came to the site to collect material for dating. The woman from the radiocarbon laboratory wanted tiny samples from bones or teeth, preferably ones with cut marks on them to show their association with human activities, so she could produce more precise age estimates.Another bloke was looking for good pieces of stalagmite from which he could get dates by Uranium series testing. So we will know more about this soon.

Unfortunately, you cannot come and join us, as the work has stopped for now. But, the good news is that it will take at least another twenty years to remove all the remains in the cave. Not that we intend to do this. They are such a rare resource and we must leave something for the future when there may be new and better techniques to discover the past.

With best wishes,
Nick

PS: I have enclosed some useful articles on various techniques, hope you find them useful.

Stone Tools

Studying stone tools is an important area of research. It is our most direct link to Ice Age people. Stone tools are quite often the only artefacts that have survived.

When a stone tool is used it is damaged. Tiny pieces of flint may be broken off the edge of the tool and the surface of the stone may be scratched and polished. These patterns of damage may provide clues about how the tool was used, whether for piercing, cutting or scraping. It may also be possible to identify on which material the tool was used, whether this was wood, bone, hide, meat, antler, or plants.

This artefact is called a 'point' because its shape suggested that it was a spear head. However, the patterns of wear damage from similar tools found in Belgium show they were used for cutting, scraping and piercing and not as spear heads.

When a stone tool is made lots of waste flakes of stone are left behind. Archaeologists attempt to fit these waste flakes back together. This shows how the tool was made and what was taken away to be used somewhere else. When this information is added to where the flakes were found on a site, it may be possible to get a picture of how people used the site and what they were doing. Unfortunately, the detailed information needed to carry out this work was not recorded during the past excavations at sites in the Creswell area.

The different types of tools found on a site provide clues about what those sites may have been used for in the past. For example, a site which contains a large quantity and variety of tools, may have been a more permanent camp. Other sites, such as Dead Man's Cave in the Creswell area, with apparently only cutting tools, may have been temporary camp sites during hunting trips where animals were butchered after a recent kill.

Although stone was used to make tools throughout the Ice Age, it is possible to recognise particular types of tools which are believed to relate to a specific culture at a certain time. This Font-Robert point is recognised as an Early Upper Palaeolithic artefact made between 28,000 and 26,000 years ago. It is similar to tools from the Gravettian cultural phase in France, Belgium, Germany and Luxembourg.

Animal Bones

Animal bones found on archaeological sites can provide an enormous range of information. Explore this bone to discover what animal remains may be tell us about the Ice Age.

Groups of animal bones can help to build up a picture about the environment. We know that certain animals prefer to live in a particular climate and that the countryside must have had certain plants, or other animals, for them to eat. A group of bones which included the combination of reindeer, wild horse, and woolly rhinoceros would suggest a cool environment while the presence of hippopotamus and narrow-nosed rhinoceros would suggest a warm environment. However, large animals can only provide a rough guide and it is the bones from animals such as rodents, bats, birds, fish and reptiles which are more useful when trying to understand environmental change. These smaller animals are more fussy about where they live and what they eat. They are therefore good at indicating environmental change.

Bones from large animals such as wild horse and reindeer are likely to be found on archaeological sites because either people or other predators were hunting them. Gathering information about the animal species, their age and sex, will provide a picture of how and when the animals were hunted. Patterns of tooth eruption and tooth wear, as well as the fusion patterns of the different limb bones, will provide information about the age of the animal at death and when in the year death may have taken place. By using the antlers of reindeer, it is possible to identify the sex of the animal being killed and the season in which hunting was taking place.
Explore the reindeer antlers found in Pin Hole

Cut marks on bone identify animals which have been butchered by people. Stone tools used to remove meat and tendons may accidentally make sharply defined cuts on the surface of the bone. It is possible to interpret whether marks on bone were made by slicing, scraping or chopping.
Explore the cut marked bone found in Robin Hood Cave

The evolution and extinction of different animals at particular times in the past can be used to roughly indicate the age of deposits in which a bone was buried. For example the discovery of hippopotamus bones at Mother Grundy's Parlour, Creswell Crags, indicates the presence of Interglacial cave deposits laid down between 130,000 and 110,000 years ago. Bone which is less than 40,000 years old can also be radiocarbon dated. Dating bone which is cut marked directly dates the presence of people.
Explore the hippopotamus bones found in Mother Grundy's Parlour

What is radiocarbon?

Radiocarbon dating is a scientific method of dating which can be used on any material which came from a living source within the last 40,000 years. Such organic materials include things often found by archaeologists such as bone, antler, ivory, teeth and charcoal.

Radiocarbon or carbon-14 (C14) is a radioactive isotope of carbon. It is produced in the upper atmosphere by radiation from the sun. It is absorbed by plants with carbon dioxide from the air. Animals eat the plants and, in turn, absorb C14 from them.

How is radiocarbon useful for dating ?

When plants and animals die they stop taking in C14 and, the C14 they have absorbed begins radioactive decay. After 5730 years only half of it will be left. This decreases to 25% after 11,460 years, 12.5% after 17,190 years, 6.25% after 22,290 years and so on. After 37,000 years less than 1% of the original radiocarbon will remain. Eventually, in about 70,000 years, it will all be gone. Given this known rate of decay, it is possible to calculate the age of an organic material by measuring the amount of radiocarbon remaining in a sample from it.

How is radiocarbon measured?

When radiocarbon was first discovered it was measured by the strength of its radioactive signal. This signal gets weaker with age and becomes increasingly difficult to detect causing uncertainties about the results, especially those from older samples. Now, most radiocarbon laboratories use accelerator mass spectrometry (AMS) to measure the amount or mass of C14 left in a sample. This allows older and smaller samples to be dated more accurately.

How does the accelerator work?

To separate C14 from all the other molecules, the sample is subjected to a massive charge of 2 million volts. This accelerates them to energy levels high enough to allow their tiny amount to be detected and measured by a meter called the mass spectrometer.

What can be dated?

In theory, the remains of anything that has lived within the last 40,000 years can be dated by the radiocarbon method. At Creswell this includes bones, teeth, antler and charcoal. The most useful samples are those which come from a known position and help to date other things by their association. For example, a piece of bone or a tooth which can be identified as coming from a particular species of animal and which also shows cut marks made by stone tools, may provide an age not just for the local presence of that species but for human activity in a known part of the site. View an example from Creswell.

What should not be dated?

Misleading dates can be obtained if a bone has been coated with a preservative or repaired with glue made from organic materials. These can add new or old radiocarbon to the sample and may make the dates too young or too old. This can be a problem when selecting samples from museum collections and old excavations as at Creswell. However, careful sampling and sample preparation can overcome this difficulty. This is not the case with charcoal. Charcoal fragments are often small and light. They move around and get mixed easily within a site. Using several small pieces of charcoal to make a sample big enough for dating may combine fragments of quite different ages so the 'date' which is obtained is just an average of the sample rather than an actual age. This can be misleading rather than helpful.

How do we know if a radiocarbon date is right?

There are two ways to check radiocarbon dates. It is possible to date things for which historians know the right answer, for example, wood from the coffin of a pharoah whose date of death is known from other sources. However, such right answers do not exist for much older, prehistoric things. The radiocarbon age estimates then have to be checked against other methods. The most important of these is tree ring counting or dendrochronology. Trees produce a new growth ring for every year of their life. These will vary in thickness according to the weather conditions. By matching up the ring patterns from tree to tree it is possible to count back through time. Samples of wood of known age can then be dated to check the radiocarbon method. When this was first done it showed that radiocarbon dates were coming out too young because the amount of radiocarbon available in the air had varied. Consequently, radiocarbon dates have to be corrected or 'calibrated' into real calendar years.

What might affect the amount of radiocarbon in a sample?

The amount of radiocarbon in the air has not always been the same. It has changed in response to solar activity (sunspots), changes in the earth's magnetic field, climatic change and, in more recent times, as a result of human activity. Detecting how much effect these variations have on radiocarbon dates is an important area of research. Which will help to improve the accuracy of the calibrations. Although long lived trees such as the Californian bristlecone pine can take us back some 10,000 years, variations in the radiocarbon supply during the climate changes of the Ice Ages are much older. New research on stalagmite (flowstone) suggests that there were dramatic variations in radiocarbon concentrations beyond 20,000 years ago and will provide better adjustment of age estimates. Similarly, the annual layers of sediment or varves laid down on the bottom of lakes can go much further back than the end of the Last Ice Age as at Lake Suigetsu in Japan, where a radiocarbon calibration curve back to 45,000 years ago has been established. However, for the moment it is best to regard radiocarbon results as age estimates rather than exact dates.

What is the difference between calibrated and uncalibrated dates?

Dates in radiocarbon years are quoted using the lower case letters bp (before present). Present is taken as 1950 AD. To allow for some imprecision in their measurement, a margin of error of plus or minus a certain number of years is quoted. For example, if a date is given as 10,000 +/- 150 bp, this means there is a 66.6% chance that its age lies between 9,850 and 10,150 radiocarbon years. A radiocarbon date calibrated to calendar years is given with the upper case letter BP or BC, without the +/- to indicate standard deviation.

Evidence from Plants

Information about the plants which grew at a site in the past can help to build up a picture about the climate, the soil conditions around a site, and provide some clues about the range of animals who were likely to have lived in that environment.

One of the main techniques used to understand about plants is the study of their pollen. Pollen is released by the male part of flowers and can survive in the ground for thousands of years. It is carried by wind, insects or animals to the female parts of other flowers.

Studying pollen from a site such as a cave is not straightforward. Some things to watch out for are:

Pollen grains can become damaged by the grinding action of sediments or eaten by microbes. Certain pollen grains are better at surviving being buried. Damage to a pollen grain can make identification extremely difficult.

Pollen is very small and can be easily washed between the deposits in the cave. This will mix pollen together from plants which grew at different times in the past. Samples of pollen trapped within hyaena droppings are less likely to have been mixed and can provide a good picture of the vegetation growing at a particular time.

Pollen is designed to be transported in different ways, by the wind, on water, or by attaching itself to insects and other animals. Pollen transported by the wind may be present in abundance while plants pollinated by insects may not be represented in the sample.

Samples of pollen taken from deposits laid down at different times can be compared to each other in order to explore changes in the plants growing close to a site. For example, the move from warm interglacial conditions to the cold of a glacial period is likely to reduce the amount of pollen from trees in a sample while increasing pollen from grasses and herbs.

Cave Sediments

Study of sediments which have built up in a cave, will help you understand what the conditions in the cave were like when they formed.

A hard cement like sediment which develops in layers in limestone caves when calcium carbonate is deposited out of water. The same process creates stalagmites and stalactites. The cave is likely to be fairly warm and wet when flowstone is forming.

The term breccia is used to describe deposits which can be hugely variable. Basically they consist of limestone and other rock fragments broken from the walls and roof of the cave and sometimes outside, mixed with sand and sometimes cemented with flowstone. Breccia builds up as a result of alternating freezing and thawing. During the winter and periods of increased cold, water freezes in rock crevices causing pieces of limestone to break away from the cave wall or roof. During warmer weather, water, which flows through the cave and between the limestone fragments washes in sand, silt and clay. Subsequent formation of flowstone can cement these pieces of limestone and the sediment between them into a solid mass. The early excavators at Creswell Crags tell us how they used explosives to remove the breccia from Robin Hood Cave.

A red sandy deposit containing rounded limestone pebbles. Bands of red clay lie within the sand. The sand may have been washed into the cave through openings in the cave roof or brought into the cave by animals. As water forms pools on the cave floor the fine particles of clay could settle out creating the clay bands.

A band of white sand lying on top of the limestone. As the limestone breaks down from the moisture in the cave and cave deposits, this pure white and yellow sand forms at the base of the layers.

Dating Techniques

How old is it? Archaeologists use various methods to date the sites and objects which they find. These can be grouped together and distinguished as relative and absolute methods of dating.

Relative dating simply allows the archaeologist to say one thing is older than another. This is usually done by placing it in a particular period such as the Lower Palaeolithic which is the oldest part of the Old Stone Age, the Devensian or Last Ice Age, oxygen isotope stage 3, or pollen zone II. These periods and the ways in which they relate have been worked out by careful study of the archaeological, geological, climatic and biological evidence. Estimates of how long these periods might have lasted can be made, but it is not possible to say exactly how old they might be unless scientific techniques can be used to provide absolute age estimates.

Absolute dating is based on the use of scientific methods to measure the age of a material in years. These techniques include: thermoluminescence (TL) which can be applied to burnt flint and pottery; Uraniumseries (U series) which can be used to date flowstone and so is useful for dating different levels within a cave and electron spin resonance (ESR) which can be used on teeth and bones. However, the most successful and widely applied technique is that of radiocarbon dating. Most of these methods, particularly radiocarbon, have been used at Creswell Crags.

This
letter was composed in 2001. It is based on documents and
publications from excavations carried out in the late 20th
century.